Automated storage and retrieval systems are frequently utilized by warehouses or other storage or fulfillment centers to store and retrieve physical items. These automated storage and retrieval systems often include computer-controlled retrieval machines that move along predetermined paths to retrieve storage bins (or other storage units) and deliver the bin to a human operator or other automated system. For example, a carousel-type automated storage and retrieval system generally places all items within a rotational carousel of holding locations. To retrieve an item from a carousel-type automated storage and retrieval system, the system rotates the carousel until the desired item is at a predetermined load and unload point on the carousel. As another example, unit load automated storage and retrieval systems typically utilize discrete retrieval machines that move vertically and horizontally along a shelf of stored goods, and include mechanisms to place or retrieve objects from the shelves.
Often, a significant portion of the space used by an automated storage and retrieval system is unusable for storage. For example, in a unit load system, each retrieval unit generally requires a portion of space between each shelf to allow the retrieval unit to access every item on the shelf. While carousel-units are more space efficient, retrieval of an item from these units requires cycling each stored item, reducing retrieval speed significantly.